The moon might favour the maintenance of colour polymorphism via disruptive selection if the different colour variants performed differently in terms of prey capture under different moonlight levels. Moonlight, however, may affect prey capture as a by side effect of its influence on prey behaviours. Here we combine data of parental provisioning and owlets' quality with one ex-situ study of grasshoppers' activity to test whether Eurasian scops owls (Otus scops) with different plumage colour and their prey are differently affected by moonlight. Food provisioning increased from new- to full-moon. However, the effect of moonlight on owlet mass gain and immune response depended on paternal coloration. On the one hand, body mass gain of nestlings of the greyest fathers increased from nights with new- to full-moon, whereas it did not change with moonlight for the brownest fathers. On the other hand, PHA response of nestlings of the brownest fathers increased with high moonlight levels during the first week of life, whereas it did not change with moonlight levels for the greyest fathers. Grasshoppers were more active at new moon than at full or waning moon. Our study provides supporting evidence that moonlight influences the behaviour of both scops owls and its prey and suggests that fluctuations in environmental conditions can modulate the advantages of morphs. These results are important because they provide a general insight into a little appreciated mechanism for the maintenance of colour polymorphism in natural populations based on the interactive effect of different environmental factors.